I just ordered a couple of cases of Fast Lane Tea -- it has more caffeine than regular drip coffee, although less than an espresso.
Admittedly, that's the exception, not the rule. You're right, most black teas are a small fraction of the caffeine of most coffees.
But I still think Dan's idea is a good one for most people -- ween yourself down to one cup a day, then switch to tea or 50%, then quit completely.
The headaches usually only last two weeks, right? (And by that, I mean not continuously, I mean on and off.)